Why do du/df and by System Monitor show used space in home differently?

$ sudo du -h -d 0 home/ 309G home/ $ df -h Filesystem Size Used Avail Use% Mounted on ... /dev/sda4 550G 309G 214G 60% /home ... System Monitor on Ubuntu shows the used space on /home is 331.1 GB Do the different ways measure the same thing? How differently do they make the measurements? Thanks.